Subterranean cavities are drilled in formations for many purposes, e.g., for extracting oil, gas, geothermal steam, water, elemental sulfur, or other valuable chemicals, for establishing research and testing bore holes, for gaining access to sites for underground storage of petroleum, and for facilitating pressurization and depressurization of subterranean formations for water flooding, and the like.
In the course of drilling and completing such wells and other holes (generically "holes"), it is often advantageous to be able to conduct a step or a process which requires the focused, localized application of a substantial amount of energy within a short time, such as by firing an explosive charge in order to move a concentrated mass in a desired direction. Typical uses, in this context, are perforating or cutting a tube, pipe or similar element, or setting a packer or plug.
In actual practice, most devices of this type in current use are electrically activated. This fact means that these are costs and delays associated with performing such steps, and these are particularly felt in the case of offshore drilling of petroleum wells. If an operator determines that the need to perform one of these steps exists, it usually means that a service person, with tools must be transported out to the rig, with an electrically operated detonator, made up on an electric cable, attached to a hoist device, lowered into the well and actuated by the visiting specialist. Expensive rig time is lost while these activities are carried out.
In recognition of the problems associated with electrical actuation of detonators from the surface, others working prior to the present inventor have devised triggering devices which can be run into a hole on a nonelectrical cable or tube, i.e., on a so-called slickline. In this regard, the present inventors are aware of the Otis ETD tool. This pressure-actuated triggering device includes battery-powered components. The triggering device is set to explode a set number of minutes after being exposed to a certain pressure, unless it has been moved in the interim. The present inventors believe this device to have shortcomings, in that its location may not be known and it may become somewhat like a "hot potato" if one has had second thoughts about whether it should fire, and must keep moving it to prevent its exploding.
The present inventors also are aware of a Twin Jet Perforator (manufacturer unstated), which is designed to be lowered into a well on a slick line, and fired by jarring down and thereby hammering a firing pin. The present inventors believe this device to have shortcomings, in that it is difficult or impossible to use in directional wells, and the presence of heavy mud in a well can cause such friction or buoyancy that insufficient acceleration of the tool can be produced, so that jarring and firing do not occur.
Also, a device which fires by jarring can prematurely, accidentally fire if dropped by a worker before it is run into the well and/or while running in the well.